Re: LF: Finbar's Compact 500kHz Vertical

[Rik Strobbe <rik.strobbe@fys.kuleuven.be>]

Dear HaJo,

I can not find any reason to believe that the same ERP radiated from 2
antennas with the same radiation pattern could produce different field
strengths.
But I am far from an antenna specialist or propagation specialist, so
maybe others can come up with these reasons.

Another thing:
At the moment we talk about skywave (ionospheric) propagation there might
be no such thing as "the best antenna". An antenna that has a
high takeoff angle may be superior to an antenna with a low takeoff angle
at shorter distances (let's say less than 1000...1500km) while the low
angle antenna will do better at long distances.

Regarding DI2AM: 
http://www.seefunk-fx-intern.de/radiobeacon/radiobeacon_en.htm
provides some figures:
- The TX has 18W input, at 75% efficiency this means about 13 W output.
- The antenna current is +/- 1.7 A, assuming 13W output the total
resistance (Rrad + Rloss) is only 5 Ohm.
- The antenna top is 35m asl. Based on the picture I estimate that the
antenna bottom is +/- 10m asl, so antenna hight is +/- 25m. The 
topload seems about twice as long as the antenna height so let's estimate
50m. According to "mmana-ga"l Rrad is 2.3 Ohm, the antenna
capacitance is 840pF (the website notes +/- 800pF, so thta's pretty
close) and the gain is 4.64dBi (2.5dBd).
- Putting 1.7A into 2.3 Ohm means 6.65 Watt radiated or +/- 12 Watt
ERP.
No wonder they are putting out such strong signal.

73, Rik  ON7YD - OR7T

At 00:51 18/06/2008, you wrote:

Dear Rik,

thank You for your contribution. Especially your last sentence is interesting for me because this may explain the success of MF-stations operating close to the coast.

Another example for exceptionally high antennas on MF for me is DI2AM, located on a museum ship in the harbour of Rostock. I have been informed that the antenna has been put up between two masts of 30 meter in height. Other favourable conditions are the low ground loss onboard of a ship and also the vicinity of water, of course. 

In general I do not have any objections to Finbar's setup; it shows that effective MF antennas can be built up on a rather small area.

But I am not yet convinced that height of MF antennas should not matter, and I will continue to observe the scenery.

OK?

73 Ha-Jo, DJ1ZB


  "Rik Strobbe" <rik.strobbe@fys.kuleuven.be> schrieb:

Dear HaJo,

But in general I feel that antenna height should not be disregarded. Even with equal ERP sky waves may develop better from higher aerials.

HW?

I guess that the only cause that 2 antennas fed with the same ERP produce different signals is a difference in the (vertical) radiation pattern, ie. the takeoff angle.

I simulated a 30m, 10m and 3m vertical (at 502kHz) with mmana_gal and found only very small differences: from 18.2 degrees for the 30m vertical to 18.6 degrees for the 3m vertical. This for a uniform ground with a conductivity of 10mS/m.

Making the ground poor (1mS/m) results in a takeoff angle of about 26 degrees (+/- a few tenths depending on the height).

Increasing the conductivity to 100mS/m reduces the takeoff angle to 12-13 degrees.

73, Rik  ON7YD - OR7T

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm for more information.

 

Disclaimer: http://www.kuleuven.be/cwis/email_disclaimer.htm for more information.